In recent years, lithium secondary batteries (typically lithium ion batteries), nickel hydrogen batteries, and other secondary batteries have increased in importance as in-vehicle power supplies and power supplies for personal computers and portable terminals. A lithium secondary battery in particular is lightweight and exhibits high energy density, and may therefore be used favorably as a high output power supply for installation in a vehicle.
A first requirement of a lithium secondary battery used as a motor driving power supply for a vehicle such as an EV (Electric Vehicle), an HV (Hybrid Vehicle), or a PHV (Plug-in Hybrid Vehicle) is favorable charging/discharging at a high rate (at least 10 C, for example). In response to this first requirement, a particle size of a compound used as a positive electrode active material may be reduced. Recently, micro-particulate positive electrode active materials having primary particles with a mean particle diameter of less than 1 μm have come into use. This type of micro-particulate positive electrode active material has a comparatively large specific surface area and is therefore suitable for high rate charging/discharging. Further, use of a positive electrode active material in which a surface of the active material particles is covered with a conductive material (carbon black or the like) has been proposed (see Patent Document 1 below, for example) with the aim of improving a conductivity of the positive electrode active material.
A second requirement of a lithium secondary battery used as a motor driving power supply is high durability. More specifically, a vehicle battery is used over a long period of time while being charged and discharged at a high rate (a high output) in a harsh environment subject to dramatic temperature variation (from a low temperature region below −20° C. to a high temperature region exceeding 60° C., for example), and therefore the battery must be sufficiently durable to ensure that an internal resistance of the battery does not increase even under such usage conditions. In response to this second requirement, the positive electrode active material particles may be held by great adhesive force in a predetermined position of a positive electrode collector (in other words, on a positive electrode active material layer). It is effective for this purpose to increase a content (a content ratio) of a binding material (a binder) included in the positive electrode active material layer.
When the content (ratio) of the binding material is increased, however, the content (ratio) of the positive electrode active material decreases correspondingly, leading to a reduction in a capacity of the battery, which is undesirable.
With regard to this point, Patent Document 1, for example, discloses a positive electrode that prevents a positive electrode active material from falling off a positive electrode collector by forming a positive electrode mixture (a positive electrode active material layer) from a binding material and a mixed positive electrode active material constituted by a positive electrode active material whose surface is partially covered by a conductive material and a positive electrode active material not covered by the conductive material. Patent Document 1 states that when the mixed positive electrode active material constituted by the positive electrode active material whose surface is partially covered by the conductive material and the positive electrode active material not covered by the conductive material is used in this manner, a degree by which the active materials are directly bound to each other by the binding material increases, and therefore the active material (particles) can be prevented from falling off the active material layer formed on the positive electrode collector.
Patent Document 2 discloses a technique for improving adhesion between a negative electrode active material and a negative electrode collector. However, the technique described in Patent Document 2 cannot be applied favorably to the positive electrode side.